Cereal grains are the most important renewable resource for food, fodder and raw industrial materials for mankind. The endosperm is the main storage organ in maize seeds, nourishing the embryo while the seed develops and providing nutrients to the seedling on germination. The endosperm contains an epidermis like layer called aleurone, which is an important source of hydrolytic enzymes required for the remobilization of stored starch and protein during germination, and covers the entire perimeter of the endosperm except for the transfer cell region.
The serine proteases are one of the best characterized groups of proteolytic enzymes in higher organism. One of the largest families of this type of enzymes is represented by the subtilisin-like family (EC 3.4.21.14). All of them share a common reaction mechanism based on the catalytic triad comprising the amino acids aspartic acid, histidine and serine. The first subtilisin-like proteinase isolated from a plant source was cucumisin, which was isolated from sarcocarp of the melon fruit (Kaneda and Tominaga 1975). In the last years, several sequences related to subtilisines have been identified in different species: Lilium, Alnus, Arabidopsis, Lycopersicon, Oryza, Hordeum, Glycine, Taraxacum . . . Despite the recent advances, the current understanding of subtilase functions in plants is still very limited. However, these enzymes have been associated to a number of physiological roles in the plant cell like microsporogenesis, symbiosis, hypersensitive response, signal transduction, differentiation, senescence and protein degradation/processing.
Currently, several examples about Subtilisin-like genes implicated in plant defence are known. The P69 Subfamily in tomato, which is formed by 6 Subtilisin-like proteases (P69A, P69B, P69C, P69D, P69E and P69F), has been implied in defence against attacking pathogen (Tornero et al. 1996; Tornero et al. 1997; Jordá et al. 2000; Jordá and Vera 2000). The P69B and P69C genes do not appear to be constitutively expressed at any stage of normal plant development. Instead, they are co-ordinately and systemically induced de novo by salicylic acid treatment or following infection with the pathogen Pseudomonas syringae (Jordá et al 1999). Jordá et al. suggested that both P69B and P69C Subtilisines may play roles as active defence weapon against the attacking pathogen or alternatively, they make take part in the remodeling or reprogramming processes of the extracellular matrix, including the cell wall, that are characteristic of pathogen afficted plants (Jordá et al 2000).
As other examples of subtilisins, the SDD1 gene encodes a Subtilisin-like protease which acts as a processing protease involved in the mediation of a signal that controls the development of cell lineages that lead to guard cell formation (Berger and Altmann, 2000). Another Subtilisin-like protein, ALE1, has been involved in the formation of a cuticle on embryos and juvenile plants (Tanaka et al. 2001). The mutant phenotypes of SDD1 and ALE1 demonstrate that at least some subtilases carry out highly specific functions in plants development. Their modes of action in the regulation of the respective developmental processes are still unknown, but SDD1 and ALE1 may be required for the generation of peptide signals, which act non cell autonomously to control plant development (Von Groll et al. 2002, Tanaka et al. 2001).